1. Field of the Invention
The present invention generally relates to a method for forming a printing plate used in a planographic (lithographic) printing operation and the like. More specifically, the present invention is related to such a plate-making method capable of recording a sharp image without being adversely influenced by laser flare, and also applicable to a process operation executed in a light room. Furthermore, the present invention is directed to a plate-making apparatus used in a plate making method, and an image recording material such as a photosensitive plate-making material.
2. Description of the Related Art
While a CTP (computer-to-plate) plate-making system (will be simply referred to as a xe2x80x9cCTP systemxe2x80x9d hereinafter) is employed as an example, conventional techniques of plate-making methods will now be described. This CTP system corresponds to such a system that while image information stored in a computer is recorded on a photosensitive plate-making material by using a laser scanner and the like, the recorded image is developed to form a printing plate. In this CTP system, printing intermediate films with respect to the respective colors employed in the known photolithography process is no longer required. As a result, this CTP system may constitute a considerably attractive system having such various merits, for instance, low cost, high-speed processing, and high image qualities.
Very recently, two different types of photosensitive plate-making materials, namely, 1) high-sensitivity photopolymer plate-making material and 2) thermal photosensitive plate-making material, have been positively developed in this CTP system in conjunction with great progresses of laser light sources used to expose these plate-making materials. In the current CTP systems with employment of the above-explained photosensitive plate-making materials, the below-mentioned problems may occur:
1) One conventional CTP system with employment of the above-explained high-sensitivity photopolymer plate-making material:
Normally, this CTP system employs as exposure light, laser light having power of approximately 100 mW to 200 mW and having wavelengths from UV (ultraviolet) to blue/green, which is produced from either an Ar+ laser or an LD-excited green solid-state laser. This CTP system may have merits of processing/printing aptitude similar to those of the conventional system using the PS (Pre-Sensitized) plate corresponding to the photosensitive plate-making material.
However, in this CTP system, since the laser light having the wavelengths from UV to blue/green and the high sensitive photopolymer are employed, the adverse influences of the laser flares are apt to be caused by laser light scattering phenomenon, laser light reflections, and laser light diffraction, which occurred in the optical system and the photosensitive planes. As a result, the resultant image qualities may be easily deteriorated. Also, in the case that the laser light of blue/green color is employed as the exposure light, both the exposure process and the developing process of the plate are required to be performed in a dark room. Namely, darkroom process operation is required.
2) Another conventional CTP system with employment of the above-described thermal photosensitive plate-making material:
A thermal photosensitive plate-making material is classified into both a thermal/negative photosensitive plate-making material and a thermal/positive photosensitive plate-making material. Since these photosensitive plate-making materials own very low sensitivities, these plate-making materials are not photosensitized by the normal illumination light, although high power laser light in the class of 1W to 10W must be irradiated to these very low sensitive plate-making materials. Therefore, these photosensitive plate-making materials can be handled in a light room.
However, since a very large oven is necessarily required in a pre-heat stage with respect to such a thermal/negative photosensitive plate-making material, a processing system thereof becomes bulky. Furthermore, this thermal/negative photosensitive plate-making material owns another problem that a latitude or a permissible range with respect to a heating temperature in a heating stage is narrow, and also very cumbersome temperature controls are necessarily required. On the other hand, in the case that a thermal/positive photosensitive plate-making material is employed in the CTP system, although a heating stage is not required, there is such a problem that a latitude of a developing process is narrow and also very strict management is required with respect to the developing conditions. Further, these two thermal photosensitive plate-making materials own a certain drawback as to stability for a long time period.
The present invention has been made to solve the above-explained problems, and therefore, has an object to provide a plate-making method capable of recording a sharp image on a photosensitive material, while an adverse influence caused by laser flare can be hardly given, and also capable of being applied to a process operation executed in a light room. Furthermore, another object of the present invention is to provide a plate-making apparatus used in such a plate-making method, and also to provide an image recording material such as a photosensitive plate-making material.
To solve the above-described problems of the prior art, a plate-making method according to one aspect of the present invention comprises the steps of: preparing a photosensitive plate-making material; scanning the photosensitive plate-making material by using modulated laser light so as to record an image on the photosensitive plate-making material, the laser light including ultra-short pulse laser light which causes photopolymerization reaction by a multiple photon absorption phenomenon at a laser-light-irradiated portion of the photosensitive plate-making material; and developing the image recorded on the photosensitive plate-making material.
Also, a plate-making apparatus according to one aspect of the present invention is a plate-forming apparatus for scanning a photosensitive plate-making material by using modulated laser light to record an image on the photosensitive plate-making material, comprising: a light source for generating laser light including ultra-short pulse laser light which causes photopolymerization reaction by a multiple photon absorption phenomenon at a laser-light-irradiated portion of the photosensitive plate-making material; a light modulator for modulating the laser light generated by the light source; and a light scanning mechanism for scanning the laser light modulated by the light modulator.
Furthermore, an image recording material according to one aspect of the present invention comprises: a photosensitive film made of a photopolymer photosensitive material, for causing photopolymerization reaction by a multiple photon absorption phenomenon when recording laser light is irradiated onto the photosensitive film; and a filter film formed on the photosensitive film, for cutting off such light having a wavelength shorter than that of the recording laser light.
As an example of the multiple photon absorption phenomenon, a description will now be made of a two-photon absorption phenomenon. This two-photon absorption phenomenon implies such a phenomenon that since a light absorption member absorbs two pieces of photon at the same time, such absorption may occur which is equivalent to energy (xc2xd wavelength) two times higher than that of actually irradiated light. In this phenomenon, for instance, when femtosecond laser light having an infrared wavelength of 760 nm is irradiated, such a light absorption occurs which is equivalent to irradiation of light having a UV wavelength of 380 nm. It should be noted that the unit of xe2x80x9cfemtoxe2x80x9d is equal to 10xe2x88x9215.
It should be understood that an occurrence probability of such a two-photon absorption phenomenon is very low, as compared with that of a one-photon absorption phenomenon in the case that light having a general light intensity is irradiated. However, since the occurrence probability of the two-photon absorption phenomenon is directly proportional to a squared value of the irradiation light intensity, if the irradiation light intensity is increased so as to considerably increase the photon density, then the induction of the two-photon absorption phenomenon becomes active. In such a case that laser light is made of a ultra-short pulse laser, the photon density becomes very large, so that peak laser power higher than, or equal to 1 kW/cm2 can be easily produced. In this case, the two-photon absorption phenomenon may occur in a practical level. Also, as previously explained, since the occurrence probability of the two-photon absorption phenomenon is directly proportional to the illumination light intensity, the two-photon absorption phenomenon may occur only at a focal position where laser light is focused.
According to the present invention, such a photosensitive material can be employed based upon the basic idea of the multiple photon absorption phenomenon. This photosensitive material owns such a sensitivity with respect to light having a wavelength equal to a half (otherwise shorter than xc2xd) of the wavelength of the irradiation laser light. In other words, such laser light having the wavelength two times longer than the photosensitive wavelength of the photosensitive material can be employed. As a consequence, the adverse influences of the laser flares which are caused by the diffraction and scattering phenomena of the laser light can be mitigated, which may especially cause a serious problem when the laser light having the short wavelength is employed. Accordingly, the following problems can be considerably solved in the non-image formed portion in the recorded image, or the recorded plate-making material. As to these problems, there are colors and films remained in the non-image formed portion, dirty portions of the plate material, thickening of halftone dots and lines, and fringes around the halftone dots.
Also, as explained above, since the occurrence probability (absorption amount) of, for example, the two-photon absorption phenomenon is directly proportional to the squared light intensity, the focal point of the laser light can be narrowed, so that the multiple photon absorption phenomenon may occur only the area in the vicinity of this focal point so as to expose the photosensitive material. As a consequence, the high resolution can be achieved.
In general, a photosensitive wavelength of photopolymer is 400 nm or shorter (namely, ultraviolet exposure range). Such photopolymer for the ultraviolet exposure range owns low sensitivities with respect to visible illumination light. As a consequence, this photopolymer may be exposure-processed in a light room, or a semi-light room where light having wavelengths higher than or equal to yellow wavelength is employed as illumination light, so that the processing efficiency of this photopolymer can be largely improved.